For years, I have been using the IP Subnet Calculator dashboard widget for quick IPv4 sanity. Jeremy Williams released this handy tool in 2005, and has been on every Mac I have owned since.

Since IPv6 is becoming more commonplace, I recently started looking for a nice simple calculator for OSX. Low and behold, I ran across Bitcricket's IP Subnet Calculator recently. The Bitcricket calculator is very simple and also contains CIDR and IPv6 functionality. I have been using the calculator for a few weeks now and highly recommend it.